Field of the Invention and Related Art Statement
The present invention relates to a method of recording bivalent signals on an opto-magnetic record medium and more particularly, to a method of overwriting bivalent signals on an opto-magnetic record medium without effecting the erasing.
The above mentioned method of over-writing the bivalent signals on the opto-magnetic record medium without erasing previously recorded signals is described in, for instance Japanese Patent Publication Kokai Sho 59-113,506, Sho 61-104,444 and Sho 62-154,347. In the opto-magnetic recording method disclosed in the Japanese Patent Publication Kokai Sho 59-113,506, the bivalent signals recorded on the opto-magnetic record medium are rewritten by changing the intensity of a laser light beam, such that an intensity P.sub.1 of the laser light beam for recording one of the bivalent signals "1" is larger than an intensity P.sub.0 of the laser light beam for recording the other of the bivalent signals "0"; Radii Rc (P.sub.1) and Rc (P.sub.0) of domains on the record medium, whose magnetization becomes zero due to the fact that the temperature of these domains is increased more than the Curie temperature by the irradiation of the laser light beam having the intensities P.sub.1 and P.sub.0, satisfy the condition of Rc (P.sub.1)&gt;Rc (P.sub.0); and in the domain upon which the "1" writing laser light having the intensity P.sub.1 is made incident, there is formed a magnetization inverse domain having a radius R.sub.w (Rw&lt;Rc (P.sub.0)), but in the domain upon which the "0" writing laser light of P.sub.0 is made incident, the magnetization is not inverted. In the present specification, said radii RC (P.sub.1 ) and Rc (P.sub.0) are also called magnetization lost radii for the sake of the simplicity.
In the known opto-magnetic recording method described in the Japanese Patent Publication Kokai Sho 62-154,347, use is made of a record medium comprising an opto-magnetic recording layer, bias magnetic layer made of a ferrimagnetic material whose compensation temperature is higher than a room temperature, but lower than the Curie temperature and a non-magnetic layer interposed between said opto-magnetic recording layer and bias magnetic layer, and the over-writing is carried out by changing the intensity of laser light between two power levels, while no external magnetic field is applied to the record medium. In the first power level, the opto-magnetic recording layer is heated near or above the Curie temperature and the bias magnetic layer is heated above the compensation temperature thereof, while in the second power level, the opto-magnetic recording layer is heated near or above the Curie temperature and the bias magnetic layer is heated below the compensation temperature.
In the above mentioned known methods, the radii Rc (P.sub.1) and Rc (P.sub.0) of the magnetization lost domains largely depend upon the temperature of the record medium, i.e., operating or room temperature. When the temperature of the record medium is high, the spot radius becomes large, so that in case of recording the bivalent signal "0", the radius Rc (P.sub.0) of the magnetization lost domain exceeds a reference value and thus the magnetization inverse domain might be formed, and in case of recording the bivalent signal "1", the Radius Rc (P.sub.1 ) of the magnetization lost domain becomes large and the radius Rw of the magnetization inverse domain also becomes large, and therefore when the temperature of the record medium is decreased in case of rewriting the signal "1", in the relevant domain into the signal "0", the radius Rc (P.sub.0) might be smaller than Rw so that there might remain a non-erased area. A similar disadvantage also occurs in the known method disclosed in the above mentioned Kokai Sho 62- 154,347. Further, use is made of the record medium utilizing the demagnetizing field; and thus, when the temperature of the record medium becomes higher and the coercive force becomes smaller, the direction of the magnetization in the record medium might be influenced to a large extent by a magnetic field leaked out of a magnetic driving device for moving an objective lens in order to effect the focusing and tracking control This results in the information signal not being recorded accurately.